OsHOX24 Modulates Grain Size and Salt Tolerance Via OsPP2C09 in Rice

Grain size and stress resistance are key determinants of high and stable yield in rice. However, the genetic and regulatory mechanisms that coordinate these traits remain elusive. Here, we report that OsHOX24 encoding a HD-ZIP I transcription factor plays a dual role in regulating grain size and salt tolerance. OsHOX24 promotes grain length and weight by enhancing cell division and expansion in spikelet hulls, while it compromises salt tolerance. Additionally, we demonstrated that OsHOX24 participates in ABA signaling and directly activates OsPP2C09 transcription by binding to its promoter, thereby jointly regulate grain size and salt tolerance. Finally, haplotype analysis revealed differentiation of OsHOX24 between indica and japonica, with the prevalent indica haplotype (Hap1) associated with superior grain size and salt tolerance. Taken together, our study not only provides novel insights into the molecular mechanisms that regulate grain size and salt tolerance, but also offers a valuable haplotype-specific target for breeding high-yielding and salt-resilient rice.